But de Castro said on Friday that Arroyo should be given time to think.
"Let's give President Arroyo a chance to think and decide for the nation," he said in a statement. "I will focus my energy on uplifting our poor countrymen."
The stock market fell at first, then bounced back to close 1.57% higher on Friday. The peso plunged to 56.444 against the US dollar, a hair's breadth away from its all-time low.
Dealers said it appeared that some investors were looking for bargains on the view that Arroyo could be forced from office, thereby ending the standoff.
Turmoil
The Philippines has been in turmoil for weeks over allegations that Arroyo conspired with election officials to fix last year's presidential elections.
The president has apologised for improperly telephoning an election official in the May 2004 election during the vote count, but denied cheating and has resolutely refused to step down.
The scandal was sparked by the release of wiretapped telephone conversations that purportedly show Arroyo plotting with a senior election official to fix a one million-vote winning margin.
Those who resigned were Trade Minister Juan Santos, Education Minister Florencio Abad, Social Welfare Minister Corazon Soliman, Agrarian Reform Minister Rene Villa, as well as senior presidential advisers Imelda Nicolas of the anti-poverty commission and Teresita Deles.
The top two revenue commissioners, internal revenue bureau chief Guillermo Parayno and Alberto Lina of customs, also signed the joint statement.
Arroyo late on Thursday called on her cabinet to resign to allow her to make a fresh start, but the six ministers said they had decided to step down earlier in the week.
"The president pre-empted our moves. This pre-emption does not change our conviction that her decisions as of late are guided mainly by her determination to survive as president," the group said.
Alert raised{mosgoogle right}
Metropolitan Manila police chief Vidal Querol said the alert was raised and additional contingents were dispatched to secure the presidential palace to prevent rowdy demonstrations.
"There are situations and we don't want the police to get caught flat-footed," Querol said, without elaborating.
The head of the Philippine armed forces, General Efren Abu, made a nationwide television address ordering soldiers to stay out of the political crisis amid reports of coup plots linked to retired officers.
US reaction
The senior US diplomat in the Philippines said his government would oppose any unconstitutional attempt to unseat Arroyo. 
US embassy Charge d'Affaires Joseph Mussomeli said in a live television interview that Washington recognised Arroyo as the legitimate president.
"Anything that goes beyond the constitution, the US government will firmly oppose," he said.
He said unacceptable options would include "a military coup, the imposition of martial law and people power in the streets".
In the past, huge street demonstrations, widely known as "people power", were used to topple presidents Ferdinand Marcos in 1986 and Joseph Estrada in 2001.
Six members of the Philippine cabinet have resigned and urged President Gloria Macapagal Arroyo to do the same to defuse a political crisis caused by vote-rigging allegations as Manila's police force went on full alert.
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